1. Field of the Invention
This invention relates to a method and apparatus for recording moving picture data encoded using a prediction encoding system, a recording medium having recorded thereon the moving picture data encoded using the prediction encoding system and a method and apparatus for decoding moving picture data encoded using the predictive encoding method. This invention also relates to a method and apparatus for judging continuous reproducibility of data encoded using the prediction encoding system.
2. Description of the Related Art
The conventional recording/reproducing apparatus for an optical disc generally includes an encoding unit and a decoding unit for companding a picture in accordance with an MPEG (Moving Picture Experts Group) system. Picture signals are recorded/reproduced as the picture is compressed or expanded using this MPEG system.
In this MPEG system, a picture constituting a moving picture (picture of a field or a frame) is encoded in accordance with a picture type of one of I-, P- and B-pictures, by way of compressing the picture.
The I-picture is in which encoding is completed within one picture, is encoded independently of other pictures. Thus, the I-picture is used as an entry point for random accessing or error restoration.
The P-picture is obtained on predictive coding from a temporally past I- or P-picture. Thus, for decoding this P-picture, the temporally past I- or P-pictures need to be already decoded.
The B-picture is obtained on predictive coding in the forward direction, backward direction or in both directions from a temporally future I- or P-picture. Thus, for decoding this B-picture, temporally past and future I- or P-pictures need to be already decoded.
Thus, in the MPEG system, pictures are compressed efficiently by inter-picture predictive coding, while random accessing can be had to the compressed moving pictures.
Also, in the MPEG system, these different sorts of pictures are compressed into a data stream made up of an optional number of pictures grouped together (group-of-pictures or GOP). The MPEG system provides that at least one I-picture be present in each GOP. This renders possible random accessing to moving pictures compressed on the GOP basis.
It is now assumed that, in the above-described conventional recording/reproducing apparatus, signals compressed in accordance with the MPEG system are to be reproduced.
On a recording medium, encoded data of a datastream shown for example in FIG. 1 is recorded. In the conventional recording/reproducing apparatus, the datastream recorded as shown in FIG. 1A is decoded to display the decoded datastream in the picture sequence shown in FIG. 1B. In FIG. 1, symbols affixed to the pictures “I”, “P” and “B” denote I-, P- and B-pictures, respectively, each suffix number indicating the display order in the GOP, that is the so-called temporal reference.
For reproducing the encoded data of the datastream shown in FIG. 1A by the conventional recording/reproducing apparatus, a picture I0 is decoded first. Since the I-picture is complete in itself, the conventional recording/reproducing apparatus can decode I0 by itself without decoding other pictures. The conventional recording/reproducing apparatus then proceeds to decode forwardly predictive-coded P2, based on the decoded I0. Since the P-picture is predictive-coded from the temporally previous I- or P-picture, it is necessary for the conventional recording/reproducing apparatus to decode I0 prior to decoding this P2. The conventional recording/reproducing apparatus then proceeds to decode the bidirectionally coded B1 based on the decoded !0 and P2. Since the B-picture is bidirectionally coded from the temporally forward and backward I- or P-pictures, it is necessary for the conventional recording/reproducing apparatus to decode I0 and P2 prior to decoding this B1. Thus, the conventional recording/reproducing apparatus decodes the encoded data of the datastream shown in FIG. 1A in the sequence of I0→P2→B1P4→B3→P6→B5→I8→B7→P10→B9→ . . . .
If, in the conventional recording/reproducing apparatus, the pictures decoded in the above sequence are to be displayed, the picture sequence is interchanged as shown in FIG. 1B to make a display in the sequence of I0→B1→P2→B3→P4→B5→P6→B7→I8→B9→P10→ . . . .
Meanwhile, in the conventional recording/reproducing apparatus, if the recording medium is random-accessible, it is possible to have random access to the encoded data recorded by the MPEG system. Thus, it is possible with the conventional recording/reproducing apparatus to skip certain pictures by first reproducing pictures up to the P-picture shown at SA and skipping the ensuing pictures to reinitiate reproduction at the B-picture shown at a point SB (picture B3). This skipping of certain pictures is hereinafter termed skipping and picture reproduction from a certain picture to a different distant picture by skipping intermediate pictures is termed skip reproduction. Also, a picture directly previous to the start of the skipping, for example, a picture SA shown in FIG. 2, is termed an out-point picture, while a picture at which reproduction is initiated after the end of the skipping, for example, a picture SB shown in FIG. 2, is termed an in-point picture.
However, if this skip reproduction is performed on the conventional recording/reproducing apparatus, there are occasions wherein temporal continuity of reproduced pictures is interrupted and lost.
If, for example, the in-point picture is a B-picture, it is necessary to previously decode an I- or a P-picture required for decoding this B-picture, in which case temporal continuity of reproduced pictures is interrupted and lost. More specifically, with reference to an instance shown in FIG. 2, since the in-point picture is B3 with the conventional recording/reproducing apparatus, at least I0, P2 and P4 need to be decoded in order to decode this B3. Thus, with the conventional recording/reproducing apparatus, no picture can be displayed while I0, P2 and P4 are decoded, such that the picture continuity is interrupted and lost.
As described above, it is not possible with the conventional recording/reproducing apparatus to achieve seamless reproduction of pictures ahead and at back of skipping on the occasion of skipping reproduction.
Although the above example is pertinent to picture compression in accordance with the MPEG system, temporal continuity in skipping reproduction is similarly interrupted and lost even in case of inter-picture predictive coding in which the difference between pictures is found in order to exploit the inter-picture correlation and the difference thus found is encoded.